We are studying the genetic control of developmental processes by focusing on a cluster of genes in Drosophila which controls the state of differentiation achieved by certain body segments of the fly. We are attempting to determine not only how each gene of the cluster evokes specific developmental changes in the fly but also how the genes of the cluster are themselves regulated and coordinated in such a way that development proceeds in an orderly manner. We are specifically attempting to determine by a variety of cytogenetic techniques how many and what types of genetic functions are included in the chromosomal region which contains the bithorax cluster bands 89El-4 of the salivary gland chromosomes. Of special interest are individuals in which there is total absence of part or all of this region; that is, homozygosity for deficiencies of the bithorax region. The over-all objective is to use the bithorax system to understand how higher organisms, including human beings, regulate their growth and development at the genetic level.